Ion Sensing with Solution-Gated Graphene Field-Effect Sensors in the Frequency Domain

Here, we examine the concept of frequency domain sensing with solution-gated graphene field-effect transistors, where a sine wave of primary frequency (1 f ) was applied at the gate and modulation of the power spectral density (PSD) of the drain-source current at 1 f , 2 f , and 3 f was examined as the salt in the gate electrolyte was switched from KCl to CaCl2, and their concentrations were varied. The PSD at 1 f , 2 f , and 3 f increased with the concentration of KCl or CaCl2, with the PSD at 1 f being the most sensitive. We further correlated these changes to the shift in Dirac point. Switching the graphene substrate from oxide to hexagonal boron nitride, led to an improved device-to-device reproducibility and a significant reduction of noise, which translated to a higher signal-to-noise ratio and resolution in sensing salt concentrations. The signal-to-noise ratio at 1 f was found to be a logarithmic function of KCl or CaCl2 concentration in the 0.1 to 1000 mM range.